The BAW resonator typically comprises a thin layer of a piezoelectric material embedded between two metal electrodes. When an RF electric field is applied at the metal electrodes, the piezoelectric material will convert the electrical energy into mechanical energy in the form of acoustic waves. The resonance frequency of the resonator will depend on the thickness of the piezoelectric material. As illustrated with reference to FIG. 1, this resonator structure may be solidly mounted on a layer of a reflective structure having alternatively high and low acoustic impedance characteristics. Such BAW resonators are commonly known as a solidly mounted BAW resonator (SBAR). Alternatively, the resonator structure may be mounted on a substrate with an air cavity. Such a structure is commonly referred to as a thin film bulk acoustic resonator (FBAR). Multiple resonators are typically arranged either in a series arm or a parallel arm of a ladder filter as illustrated with reference to FIG. 2.
Each resonator exhibits a series resonant frequency and a parallel resonant frequency. The parallel resonant frequency occurs at a slightly higher frequency than the series resonant frequency. At the series resonant frequency, the resonator behaves as a low impedance element. At the parallel resonant frequency, the resonator behaves as a high impedance element. Thus, in the design of the ladder filter illustrated by way of example with continued reference to the configuration of FIG. 2, it is desirable to set the series resonant frequency of the series arm resonators and the parallel resonant frequency of the parallel arm resonators at the nominal center frequency of a bandpass filter. Such a ladder filter design configuration will yield low insertion loss across the passband that is normally correlated to the difference between the series and parallel resonant frequencies of the resonator.
In order to achieve a smooth passband across a filter bandwidth, it is desirable to have minimal spurious responses. The resonator structure of FIG. 2 exhibits a spurious response due to the excitation of lamb waves at the electrode edges. One method of suppressing spurious Lamb modes in BAW devices is the application of additional mass loading to the top electrode of the resonator in a ring-like region at the boundaries of the active area as shown in the cross-sectional view of FIG. 3 and top view of FIG. 3a. By way of example, U.S. Pat. No. 6,548,943 to Kaitilla et al. discloses use of the border region above the top electrode to control the spurious modes. As disclosed in Kaitilla, the border region separates the resonator into two zones, an inner center zone and an outer border region area. Adjusting the thickness and the width of the frame-like or ring structure has been shown to reduce the spurious responses between the series resonant frequency (fs) and the parallel resonant frequency (fp) of the resonator.
By way of further example and with reference to FIG. 4a, a Smith Chart plot of the resonator illustrates a width of the ring structure not wide enough to provide an optimum or desirable spurious suppression between the fs and fp. FIG. 4b illustrates one case in which the border region width is wide enough to suppress the spurious modes between fs and fp. However, it exhibits a strong resonant mode below fs. FIG. 4c illustrates the spurious modes characteristics as a function of the border region width, wherein Curve 1 quantifies the resonant mode and illustrates results with a relatively small width and wherein a resonant mode below fs is very well suppressed. However, it has high spurious modes falling between fs and fp as shown in Curve 2, quantifying the strength of the spurious mode (so called “dimples” of FIG. 4a between fs and fp) when the border region width is large enough as shown in point B on Curve 2 to provide maximum suppression of spurious modes between fs and fp. The resonant mode below fs begins to increase sharply. Thus, the ring structure provides a means to suppress passband spurious modes, but undesirably introduces a strong resonant mode below fs. As will be discussed with regard to the invention, the resonant mode is due to the parasitic electric field arising in the ring region directly under the top electrode ring region and the bottom electrode as illustrated with reference to FIG. 3. It is desirable to implement a border ring or frame-like structure wide enough to suppress spurious modes, but with only a limited parasitic electric field.